1. Field of the Invention
The invention relates to a method for classifying sand.
Classified sand, that is, sand containing specified proportions of different-size sand grains per unit of volume, is normally produced by introducing a mixture of sand and water at a predetermined rate into a "classification tank". The tank is partially filled with water, and when the sand/water mixture is introduced into the tank through an inlet pipe, the grains of sand will sink to the bottom of the tank. The grains of sand precipitate in the bottom of the tank at graduated distances from the inlet, depending upon their weight, the heavier particles precipitating nearest the inlet and the lighter particles gradually farther away. There are a number of valves provided at the bottom of the tank, and when the valves are opened the overlying sand can pass through the valve into a common outlet pipe. Depending on which valves are opened, and for how long, sand of a predetermined composition--"classified sand"--is thereby produced.
2. Prior Art
Recent studies have shown that substantial savings of cement and a concrete of substantially improved castability can be obtained if the sand that is utilized contains a certain quantity of fines, i.e., grains of sand between 0.002 mm and 0.006 mm in size. Such particles, however, are so lightweight that with the classification processes utilized today, much of this material is simply lost, being washed out of the tank together with the overflow water.
To recover the fines from the overflow water usually requires the use of costly auxiliary equipment such as cyclones and centrifuges.
Another drawback of the classification processes used today is that they require an enormous quantity of water.